A Device-to-Device (D2D) technology enables neighboring terminal devices to transmit data by direct links within a close range, instead of forwarding data via an access network equipment. D2D communication and a cellular system may share licensed band resources to form a unified hybrid cellular-D2D network. In this hybrid network, a part of terminal devices can still work in a cellular communication mode, that is, communicate with other terminal devices through an access network equipment, and a part of terminal devices can work in a D2D communication mode, that is, directly transmit data with other terminal devices through direct links between the terminal devices and the other terminal devices.
Moreover, a D2D relay can be used to assist data transmission between an access network equipment and a terminal device. In this case, the D2D relay and the D2D terminal adopt a D2D communication mode, and the D2D relay and the access network equipment adopt a cellular communication mode. The D2D relay receives and forwards data in a half-duplex mode, and changes its mode during the receiving and forwarding process.
In the related technology, an Evolved Packet System (EPS) bearer is set up between a D2D relay and a cellular network, and a D2D terminal is invisible to the cellular network. In this case, the D2D relay needs to perform Internet Protocol (IP) Network (NET) traversal when relaying data transmitted between the D2D terminal and the cellular network, and the process is rather complex.